HEAT STRESS IN COWS – EFFECTS AND CONSEQUENCES

Heat stress typically results when the heat load of the air temperature, the associated humidity, and the solar radiation at the time prevents the ability of the cow to effectively dissipate the heat generated from production and metabolism. This usually occurs above the temperature-humidity index value (THI) of 72, which then results in reduction in feed intake and milk production (to a large extent due to the lower feed intake), increased water intake, poorer colostrum quality, lower birth weight of calves, reduction in fertility and immunity, and with major implications to animal welfare. There are also longer term effects depending on the duration of the adverse climate. General signs of heat stress in the cow include increased respiration rate, panting, lethargy and increased salivation. Apart from the lower milk production because of less feed intake, other contributing factors include reduced blood flow to internal organs, acidosis, lower gastro-intestinal tract activity, altered nutrient metabolism and hypersensitivity to insulin which then blocks fat catabolism. This results in heat stressed cows becoming increasingly dependent on glucose for their energy needs while less is channelled to the udder for milk production. 

Alleviation of heat stress through nutritional means should address the following: (1) prevent fat mobilization and increase glucose metabolism; (2) acidosis risk is higher due to lower saliva production because of lower feed intake and panting, and ketones if fat is mobilized, thus feed extra buffers and maintain a DCAD balance of 300-400 meq per kg DM with increases in Na, Mg, Cr and Zn; (3) consider certain feed additives which have shown variable alleviation such as yeast and yeast cell wall, probiotics, niacin, betaine, feed enzymes and ionophores. Heat abatement management strategies include shade (difficult with pasture irrigation systems), proper ventilation in the parlour and holding pens, evaporative cooling sprinklers, fans, misters to cool the environment and convective cooling. Adjusting the feeding and cow handling routines are also effective heat mitigating strategies, e.g. by feeding smaller portions at more intervals if practical and allowing cows to feed more at night. It is vital to support acclimatization of the animals over the medium to long term, also by selecting within the herd for those cows that cope better than others.